1. Field of the Invention
This invention relates to improvements in a fuel cell. More particularly, this invention concerns a fuel cell using an acidic electrolyte such as a phosphoric acid fuel cell or a polymer electrolyte fuel cell which uses an electrode catalyst in the cathode thereof and a method for producing electrodes for use in the fuel cell.
2. Description of the Related Art
In recent years, the fuel cell has been attracting growing attention as a highly efficient energy conversion device. The fuel cell generally is formed of a plurality of elementary cells comprising an electrolytic layer containing ion seeds which contribute to the electrode reaction and hold two electrodes severally of positive and negative polarity, which are capable of transferring electrons without being degraded by the electrode reaction and also capable of migrating reactants and holding the electrolytic layer therebetween. This fuel cell is completed by superposing such elementary cells through the medium of intervening current collector plates. Since the electrodes mentioned above are intended to afford a site for generating an electromotive reaction, they are generally formed of a porous substance and made to contain a catalyst which contributes to the electromotive reaction. Thus, they function as a catalytic electrode.
The fuel cells constructed as described above are classified by the kind of electrolyte into phosphoric acid fuel cells, polymer electrolyte fuel cells, molten carbonate fuel cells, etc. The fuel cells using an acidic electrolyte such as the phosphoric acid fuel cells and the polymer electrolyte fuel cells are each composed of a pair of opposed gas-diffusion electrodes, i.e. a cathode and an anode, and the layer of an electrolyte retaining electrolytic phosphoric acid or a proton-conductive solid polymer electrolyte that is held by the pair of electrodes mentioned above. As the matrix for supporting the gas-diffusion electrode mentioned above, a generally porous carbon electrode substrate having carbon as a main component thereof is used. The electrode substrate is so constructed as to have a catalytic layer such as of platinum carried on the surface thereof. The gas feed layers incorporated in the electrode matrix supply hydrogen gas to the anode and oxygen gas to the cathode.
When the hydrogen gas is fed to the anode and the oxygen gas to the cathode respectively as described above, the catalytic layer of the anode generates a reaction of the following formula: EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 1)
The electrons arising from this reaction perform work while flowing through an external circuit, then reach the cathode, and induce the catalytic layer of the cathode to generate a reaction of the following formula: EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O (2)
and entail evolution of water. At this time, the H.sup.+ ions occurring in the electrolyte migrate from the anode to the cathode.
The most profound problem to be encountered in contemplating the commercial operation of a power generating plant using such acidic electrolyte type fuel cells as described above resides in decreasing the cost. To be specific, since the price of the fuel cell stacks accounts for a large proportion of the cost of the power generating plant, the decrease in the cost of these fuel cell stacks is the gravest task of all. As one measure for the accomplishment of the cutting of the cost, the improvement of performance of fuel cells (increase in the output per unit surface area) can be cited besides the improvement on the technique of production. When the output is doubled, for example, the cost of the fuel cell stacks can be decreased roughly to 1/2 and the total cost of the power generation plant can be significantly lowered.
For the purpose of enhancing the performance of an acidic electrolyte fuel cell, particularly a phosphoric acid fuel cell, the enhancement of performance of the electrodes, especially the improvement of the cathode, in the fuel cell is highly effective. To be more specific, the activating polarization of the oxygen-reducing reaction in the cathode is notably larger than the activating polarization of the hydrogen-oxidizing reaction in the anode. For the purpose of lowering the activating polarization of the oxygen-reducing reaction, measures for the improvement of the electrode catalyst in the cathode such as, for example, the adoption of an alloy catalyst (as disclosed in Japanese Unexamined Patent Publication No. 163,746/1987, for example) have been proposed to date. These measures, however, are not fully satisfactory.
With a view to cutting the cost of the acidic electrolyte type fuel cell, the desirability of lowering the activating polarization of the oxygen-reducing reaction in the cathode thereby improving the quality of the cathode and consequently promoting the enhancement of the performance of the fuel cell has been gaining increased recognition.